Blown by cold air to bring wintry weather, snow to some in the eastern US

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A blast of arctic air descends south across the Lower 48, bringing a wintry feel and temperatures 10 to 20 degrees below average to nearly 200 million Americans. In addition to the coldest air in autumn so far, many will even see the first snowflakes of the season. Accumulating snow is likely in parts of the Great Lakes, with up to 2 feet projected in the Upper Peninsula of Michigan.

Forty million people from Kansas to North Carolina are also under freeze warnings, with another 40 million included in freeze watches or frost advisories. The onset of cold air in the early season is likely to mean an end to the growing season and kill vulnerable or exposed plants.

A few thunderstorms are even possible along the shores of the Great Lakes. Lake Erie, meanwhile, can anticipate scattered waterspouts as cold upper-level air moves overhead.

The air mass originates in the High Arctic north of Russia and Alaska, after spending the past week relaxing over Canada on its way to the Lower 48. The cold snap will last three or four days, with a welcome reprieve as the heat returns back next weekend.

How hard will the winter be? Six organizations issue forecasts.

In some areas, a garnish of fresh snowfall will decorate fall foliage nearing peak color, as in the Alleghenies and Appalachians. In the Upper Peninsula of Michigan, where a full-blown blizzard is underway, more significant snowfall will hit the fully leafed trees, causing damage and power outages.

What causes the cold

The abnormally cool weather can be pushed into an upper-level low or a pocket of high-altitude, low-pressure, spin cold air that lies in a dip in the jet stream. It is parked over the Great Lakes at the moment and will spend about 48 hours in space and strength. This counter-clockwise flow will swirl down frosty Canadian air and spill it all the way down to the Mid-South.

Using NOAA’s HYSPLIT model, which simulates the paths traced by air parcels, meteorologists can conclude that the air mass hovering over the Great Lakes and Northeast was actually over Siberia on October 10. Since then, it has passed over the Chukchi Sea north of Alaska and through Canada, making a small loop before sliding south near Hudson Bay.

The stubbornly cool air mass, with winds out of the west-northwest, will last through Thursday.

Temperatures will be between 15 and 20 degrees below average for days. In Minneapolis, gusty winds and high temperatures around 37 are in the cards for Monday. The average is closer to 58 degrees. The next few days don’t get much better.

Chicago was headed for a high of 40 degrees for Monday, with a chance of a patchy rain or snow shower in the afternoon. Tuesday and Wednesday will be in the mid to upper 40s. Averages are closer to the lower 60s this time of year.

Indianapolis, Columbus and Detroit will spend several days in the 40s before a gradual warm-up by the end of the week. Nashville, Raleigh and Washington, which should be in the upper 60s to around 70, will be in the mid-50s midweek with lows in the 30s.

DC area forecast: Blow of cool air arrives Tuesday; warmer at the weekend

A number of overnight low temperature records may be challenged or broken. Most areas across the Dakotas, Nebraska and Minnesota will drop into the teens or lower 20s Tuesday morning, including Aberdeen, SD, where a morning low of 10 degrees is expected. That would break the record of 12 degrees set in 1930 and would also mark the coldest temperature recorded this early in the season since 1937.

Lake-effect snow piled up on the Upper Peninsula of Michigan, where forecasts of up to 20 inches could be realized early Tuesday. Cold air sweeping out of Canada picked up heat and moisture from the relatively warmer waters of the Great Lakes – the perfect recipe for factory-like snow production. Winter storm warnings were in effect.

Snowfall totals in Marquette, Mich., could rank among the top 10 highest ever during October, with 4 to 10 inches predicted.

A winter weather advisory was also in effect for Porter and Jasper counties in Indiana, where a localized filament of lake-effect snow is possible. The National Weather Service in Chicago noted that “there is a higher degree of uncertainty than usual with the snow forecast.” Subtle changes in wind direction will have a significant impact on where the snow band materializes and drifts, assuming it does at all.

Significant accumulations of half a foot or more are possible along the eastern shore of Lake Erie, and especially from Erie to Buffalo along Interstate 90. An isolated rumble of thunder cannot be ruled out.

Multiple water discharges are also possible on Lake Erie itself thanks to converging or gathering winds and a sufficiently unstable atmosphere. Warm water temperatures will create lift and encourage surface eddies to be stretched vertically by convective updrafts, or pockets of air, rising into individual storm cells.

A few inches of snow is also possible atop the Tug Hill Plateau in western New York state. Additionally, the western slopes of the Alleghenies in West Virginia and the Panhandle of Maryland may also see a dusting to an inch or two of snow due to “upwelling snow,” or air that carries moisture forced up the mountains. A warm-up is expected around Friday.

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